1. Field of the Invention
The present invention relates to an optical disk apparatus for performing record/regeneration optically, especially, to an access control apparatus for an optical disk, which can be responsible for a case even if a track pitch in the optical disk changes.
2. Description of the Prior Art
Currently, with recent advancement in laser technology, an optical disk apparatus having a high density, a large capacity, and a possibility of random accessing becomes noteworthy, and apparatus relating to this field have been progressed.
As for an access apparatus in the field of optical disk apparatus, the apparatus shown in FIG. 10 is disclosed in JP-A-62-231430 (1987) or in Nikkei Electronics (Apr. 18, 1988, pages 211-224).
Referring to FIG. 10, an optical disk 1 is driven by a spindle motor 2 and rotates so as to pursue a desired information track by being tracked in a radius direction of the optical disk with an optical head which is mounted on a positioner 3. The optical head performs recording and regeneration of information by projecting a light spot on the information track which is provided on the optical disk 1. An output from the positioner 3 is taken into a tracking error generator 4 which generates a tracking error signal corresponding to an amount of relative displacement of the track and the light spot on the optical disk 1, and an output from the tracking error generator 4 is taken into a following servo circuit 5, a velocity detector 6, and a track counting circuit 10.
The following servo circuit 5 supplies an output back to the positioner 3 so as to make the light spot pursue the desired information track corresponding to the tracking error signal from the tracking error generator 4, and accordingly, a tracking control loop is composed. The velocity detector 6 detects a relative velocity of the positioner 3 to information track in accordance with the tracking error signal. The track counting circuit 10 is supplied a number of tracks to run from an input device 11 when the optical head must access to a desired information track, and subtracts one from the number of the tracks to run at each time the light spot crosses over one track, and generates a timing signal at a best position where the optical head must touch on the desired information track at the time when the number of tracks to run becomes zero. The timing signal generated by the track counting circuit 10 is taken into a reference velocity generator 7 and a selector 9. The reference velocity generator 7 gives a relative velocity to the tracks of the positioner 3 in correspondence with the number of tracks to run to the desired objective track, the outputs from the reference velocity generator 7 and the velocity detector 6 are taken into an error amplifier 8, the error amplifier 8 amplifies an error between the output from the reference velocity generator 7 and the output from the velocity detector 6, and returns the amplified error to the positioner 3. Owing to the returning of the output from the error amplifier 8 to the positioner 3, a velocity control loop is composed. The selector 9 selects either of two loops, a tracking control loop and a velocity control loop, in accordance with the timing signal from the track counting circuit 10.
FIG. 11 is a block diagram showing a concrete composition of the tracking error Generator 4 by a prior art, which is composed of photoelectric converters 401a, 401b, current-voltage converters, 402a, 402b, and an error amplifier 403. The photoelectric converters, 401a, 402b, receive reflected lights from the optical disk 1 at the light spot and convert the lights to electric currents, and the electric currents as outputs from the photoelectric converters, 401a, 402b, are converted to voltage signals by the current-voltage converters, 402a, 402b. The error amplifier 403 calculates the error between the two outputs from the current-voltage converters 402a, 402b, amplifies the error, and outputs a tracking error signal 41 in correspondence with an amount of dislocation in the position of the light spot on the optical disk 1.
FIG. 12 is a block diagram showing a concrete composition of the reference velocity generator 7 by a prior art. The reference velocity generator 7 is composed of a reference velocity data memory 701 which outputs an instructing value for velocity in correspondence with the number of tracks to go to the objective track shown on a track counter in the track counting circuit 10, and a digital to analog converter 702 for converting digital outputs from the reference velocity data memory 701 to analogous signals. Generally, the instruction from the reference velocity data memory 701 has a value proportional to a square root of a distance to the objective track, and the instruction is prepared so that the velocity on the objective track becomes approximately zero. And, the digital to analog converter 702 converts the outputs from the reference velocity data memory 701 to an velocity instructing signal in correspondence with the number of tracks to go to the objective information track and outputs to the error amplifier 8.
FIG. 13 is a block diagram showing a concrete composition of the velocity detector 6 by a prior art, and an example of the period measuring type velocity detector which detects a relative velocity to tracks of the positioner 3 by measuring a period of crossing over the tracks. Referring to FIG. 13, the velocity detector 6 is composed of a waveform shaping circuit 601, an oscillator 602, a counter 603, a period-velocity convertible memory 604, and a digital to analog converter 605. The wave form shaping circuit 601 generates the tracking error signal from the tracking error generator 4 pulses, the counter counts the output from the oscillator 602 during a period of the pulsed tracking error signal, and the period-velocity convertible memory 604 converts the information indicating the period supplied from the counter into a value proportional to the velocity.
In accordance with the period measuring type velocity detector which is composed in a manner as described above, the counter 603 counts the outputs from the oscillator which oscillates with a designated period during a period of the tracking error signal which is detected by the tracking error generator 4 when the positioner 3 crosses over a track. When the period for crossing over the track is short, that is, when the relative velocity to the track of the positioner 3 is fast, a counted value becomes small, and when the period for crossing over the track is long, that is, when the relative velocity to the track of the positioner 3 is slow, the counted value becomes large. That means, the counted value by the counter 603 is reversely proportional to the velocity. Accordingly, the period-velocity convertible memory 604 is made to supply a value as an output to the digital to analog converter 605 which is reversely proportional to the counted value taken as an input. The digital to analog converter 605 converts the value to a value proportional to the relative velocity to the track of the positioner 3, and outputs to next stages.
FIG. 14 is a block diagram showing another composition of the velocity detector by a prior art, and an example of a differential type velocity detector which detects a relative velocity to the track of the positioner 3 by differentiating the tracking error signal. Referring to FIG. 14, the velocity detector 6 is composed of a differential circuit 610 which differentiates the tracking error signal supplied from the tracking error generator 4 as an output, an invertor 611 which reverses an output from the differential circuit 610, a timing generator 612 which outputs a timing signal for indicating a linear portion in the tracking error signal corresponding to a direction of inclination for the linear region portion of the tracking error signal, switches 613, 614, and a hold circuit 615.
Referring to FIG. 15, operation of the differential type velocity detector is explained.
When the positioner 3 moves with a constant velocity, a tracking error signal 41 which is detected by the tracking error generator 4 prepares differential waveforms 42, 43, with the differential circuit 610 and the invertor 611. On the other hand, the timing generator 612 prepares timing signals 46, 47, based on threshold values 44, 45, which indicate the linear region portion of the tracking error signal, and transmits the timing signal to the switches 613, 614. Each of the switches 613, 614 closes when the timing signal is in a high level, and opens when the timing signal is in a low level. Accordingly, the switch 613 transmits only the vicinity of a positive peak in the differential waveform 42 to a subsequent step by the timing signal 46, and the switch 614 transmits only the vicinity of a positive peak in the differential waveform 43 to a subsequent step by the timing signal 47. Consequently, if a frequency of the tracking error signal is constant, the hold circuit 615 outputs a nearly linear value 48. The above described output 48 takes a large value when the frequency of the tracking error signal is high and a small value when the frequency of the tracking error signal is low because the output is prepared by a differential operation. That means, the above described output 48 indicates a value proportional to a relative velocity to the track of the positioner 3.
Currently, regarding to an optical disk apparatus, improvements of a linear recording density and a track density for increasing a memory capacity of a disk by adoption of a laser having a shorter wave length than the length in a practical use at present is under consideration. However, changing of a track pitch causes a change of detective sensitivity of the tracking error generator which detects a relative difference in positions of the track and the light spot. And, if the track density and the track pitch change, the detective sensitivity of the above described velocity detector must be also changed because the velocity detector detects a relative velocity of the positioner to the track based on the tracking error signal. Furthermore, the reference velocity generator instructs a relative velocity of the positioner to the optical disk in correspondence with a value of the track counter in the counter which indicates a number of remaining tracks to go to the objective track. Therefore, if the track density changes, a relationship between the value of the track counter and a distance to the objective track changes, and accordingly, an instruction for velocity in correspondence with the distance to the objective track is also changed.
As above explained, when a disk having a large track density is used in an access control apparatus corresponding to a disk having a conventional track density, or when a disk having a conventional track density is used in an access control apparatus corresponding to a disk having a large track density, characteristics of the tracking control loop and the velocity control loop are changed, and consequently, such a problem as the loops become unstable is caused. That means, a problem that an interchangeability between the disk having a conventional track density and the disk having a larger track density than that of the conventional one can not be realized.